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If the blower chart for a psc motor blower rate the required 1000 cfm of air at the standard SP of .5, then at .7-.8 SP that psc blower will only be providing about 700-800 cfm of air. With a psc motor, this can be a problem during humid days when a wet coil from low airflow will create even more resistance which will lower the cfm ability even more until the coil eventually freezes up.

With the same system using a VS blower, the VS motor set at 1000 cfm is going to continually ramp up to provide that 1000 cfm of air. There is no SP differential chart for a VS blower because the motor continues to attempt to achieve the amount of air it is programmed despite varying SP conditions...within reason.

Some have a derate percentage to use above X amount of static.

If a current system with a PSC blower is moving 800 CFM at a .75" static. Then increasing CFM to 1000 would raise the static to roughly 1.17"
They end up using about the same amount of electric in that type of an install, and making the system loud.
VS blowers don't have CFM ratings at that high of a static.

If a current system with a PSC blower is moving 800 CFM at a .75" static. Then increasing CFM to 1000 would raise the static to roughly 1.17"
They end up using about the same amount of electric in that type of an install, and making the system loud.
VS blowers don't have CFM ratings at that high of a static.

This is what I was referring to when I mentioned the psc motor blower performance chart. If the psc motor that is installed in a particular blower is not rated to perform above a certain SP, then it is better to use a larger HP motor. This is essentially what equipment manufacturers do with high static blower furnaces. I believe the most that I have seen for SP for VS motors is .8 SP. A system that is that close to the edge can have issues with just a dirty filter.

My static for my VS is at 1.1....so i am having duct work done. I've never noticed any huffing though nor any "oil-canning" (ducts making loud noice from bowing in and out due to the pressure). My duct guy said the VS motors can run at 1.5 SP (of course I don't believe him). He is really good with sheet metal....but does not appear to possess the knowledge of the Pro's on this site.
Key1

Sorry if I'm asking dumb questions, but is the static pressure test something that can be done fairly easily by the contractor while he's here the first time, or is that something that needs special equipment and time to set up? Also I've got the flexible duct work installed, is that going to make a difference in terms of the SP?